Apparatus for electrostatic reproduction using plural charges

ABSTRACT

An improved process for electrostatic reproduction. A transparent charged sheet of insulating material, such as a thin insulating film bearing a uniform electrostatic charge on one side thereof, or an electret, is placed against an electrostatically charged photoconductive surface on a suitable substrate to form a temporary composite. The photoconductive surface is then exposed to a light pattern and the free surface of the transfer sheet is developed to provide a visible image corresponding to the light pattern. This image is fixed on the transfer sheet or transferred to a receiving sheet after the transfer sheet has been removed from the photoconductive surface. Further copies can be made by reapplying the transfer sheet to the photoconductive surface and redeveloping the free surface of the transfer sheet when in place on the photoconductive surface. 
     Real electrostatic images can be provided on the free surface of the transfer sheet by charging it to a constant voltage, as with a constant voltage-variable current corona device, during or after light exposure. Multiple copies of the image can be obtained by placing the real electrostatic image side of the transfer sheet on a grounded conductor bearing a thin blocking layer and toner developing the opposite surface of the transfer surface of the transfer sheet. 
     In other embodiments, by simultaneously exposing and developing from opposite sides of the composite, high decay rate, but transparent, photoconductive materials can be used. In another method, the sandwich is simultaneously charged to constant voltage and exposed.

This is a division of application Ser. No. 343,621, filed Mar. 21, 1973,now U.S. Pat. No. 3,843,361.

FIELD OF THE INVENTION

The present invention generally relates to image reproduction and moreparticularly to improved processes for electrostatic image reproduction.

BACKGROUND AND SUMMARY OF THE INVENTION

In a typical method of xerographic reproduction, electric charges aredeposited on a photoconductive surface by a corona discharge, afterwhich the charged photoconductive surface is exposed to a light patternto form a latent electrostatic image thereon. This latent image is thenrendered visible by applying toner, which may be electrostaticallycharged powder or the like, directly to the photoconductive surface sothat it adheres thereto in the latent image-bearing areas throughelectrostatic attraction. The resulting visible image is then fixed to apermanent image, as by heating or the like, to fuse it in place eitherdirectly on the photoconductive surface or after print-off to a suitablecopy sheet, such as paper.

The abrasiveness of toner powder results in wear of the relativelyexpensive permanent photoconductive layers used in copying machines,thereby degrading the quality of copies and ultimately requiringreplacement of the photoconductive layers. Moreover, difficulties areencountered in fully transferring the visible image from thephotoconductive surface to the copy and of keeping the toner powder fromimage-free areas. Gradual toner powder build-up on and around thephotoconductive surface also degrades the copy quality, sinceinadvertent toner transfer to copies causes the copies to appear grayand splotchy in background areas, reducing contrast and definition.

An additional problem with such reproduction procedures is that aseparate exposure of the photoconductive surface is needed for eachcopy, that is, multiple copies cannot be made from a single exposure ofthe photoconductive surface. In addition, multiple copies bearing two ormore different toner colors cannot be made.

Certain newer xerographic processes have been developed to overcome someof the foregoing drawbacks but are usually relatively complicated andare not adapted for use in simple, inexpensive copying machines.

In copending U.S. patent application, Ser. No. 215,873, now U.S. Pat.No. 3,820,985, filed Jan. 6, 1972 by Joseph Gaynor, Terry G. Anderson,Walter Hines and Len A. Tyler, and assigned to the present assignee, animproved simple electrostatic copying process is provided which permitsmultiple copies from a single exposure and allows multiple colorcopying. In that process a thin insulating film is disposed on anelectrostatically charged photoconductive surface. An electrostaticimage induced on the free surface of the film is developed withelectroscopic toner which can be transferred to a copy sheet. Thephotoconductive surface is thus protected from the abrasive effect ofthe toner particles.

The present invention provides improvements in contrast and resolutionover the Gaynor et al process described above and provides all of itsadvantages and others. In accordance with the present process, oruniformly charges one side of a transparent sheet of insulatingmaterial, such as a thin insulating film as described in the aforenotedGaynor et al application, or an electret, and places the charged sideagainst an electrostatically charged or uncharged photoconductivesurface on a suitable substrate to form a temporary composite. Thephotoconductive surface is then exposed to a light pattern and the freesurface of the transfer sheet is developed to provide a visible imagecorresponding to the light pattern. This image is fixed on the transfersheet or transferred to a receiving sheet. The transfer sheet should beremoved from the photoconductive surface when the subsequent treatmentmay affect the electrostatic image (e.g. if fixing or transfer isthermal) or for mechanical facility. In the transfer mode, furthercopies can be made by reapplying the transfer sheet to thephotoconductive surface, if it has been removed, and redeveloping thefree surface of the transfer sheet when in place on the photoconductivesurface.

Real electrostatic images can be provided on the free surface of thetransfer sheet by charging it to a constant voltage, as with a constantvoltage-variable current corona device, during or after light exposure.The real electrostatic image can be used to provide multiple visiblecopies of the image without having to recontact the transfer sheet withthe photoconductive surface. This is accomplished by placing the realelectrostatic image side of the transfer sheet on a grounded conductorbearing a thin blocking layer and developing the opposite surface of thetransfer sheet to provide a visible toner image, transferring the tonerimage to a copy sheet, and repeating the developing and transferring toprovide the desired number of copies.

The present process can also be successfully used when thephotoconductive surface is extremely light sensitive and has a high darkdecay rate. In one method, the photoconductive surfaces and substratesare transparent. After application of a precharged transfer sheet, thecomposite is simultaneously exposed and developed (from opposite sidesof the composite). In another method, the sandwich is simultaneouslycharged to constant voltage and exposed.

The present methods provide single or multiple copies in one or aplurality of colors from a single exposure. Moreover, thephotoconductive surface is fully protected from wear and contrast lossby the toner. Importantly, the copies are full, sharp, clear and of highcontrast and resolution. The process can be carried out in a variety ofmodes to suit individual needs, all of which modes are characterized, inpart, by the use of a precharged transfer sheet of thin insulating filmor an electret. The photoconductive surface can be precharged oruncharged. Relatively permanent real electrostatic images can be formedand highly light sensitive, high dark decay rate photoconductors havingwide spectral sensitivity can be used efficiently. Further features ofthe present process are set forth in the following detailed descriptionand accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically depicts a photoconductor-transfer sheet compositeduring exposure in accordance with a first embodiment of the presentprocess;

FIG. 2 schematically depicts the composite of FIG. 1 after exposure butbefore developing;

FIG. 3 schematically depicts a photoconductor-transfer sheet compositeduring exposure in accordance with a second embodiment of the presentprocess;

FIG. 4 schematically depicts the composite of FIG. 3 after exposure butbefore developing;

FIG. 5 schematically depicts a photoconductor-transfer sheet compositeprior to exposure in accordance with a third embodiment of the presentprocess;

FIG. 6 schematically depicts the composite of FIG. 5 during simultaneousexposure and developing;

FIG. 7 schematically depicts a photoconductor-transfer sheet compositeduring exposure in accordance with the present process;

FIGS. 8A and 8B schematically depict the composite of FIG. 7 duringcharging of the free surface of the transfer sheet to constant positive(8A) or negative (8B) voltage to provide a real electrostatic image;

FIGS. 9A and 9B schematically depict the transfer sheets of FIGS. 8A and8B, respectively, after real electrostatic image formation thereon andduring developing thereof on a grounded conductor;

FIG. 10 schematically depicts the transfer sheet of FIG. 8A, after realelectrostatic image formation thereon and during developing in aninverted mode on a grounded conductor bearing a thin blocking layer.

FIG. 11 schematically depicts heat fusion of a toner image on a transfersheet; and

FIG. 12 is a diagrammatic view of a mechanism for accomplishing anembodiment of the invention wherein toner is transferred from thetransfer sheet to a receiving sheet and the transfer sheet is reappliedto the photoconductive surface.

DETAILED DESCRIPTION FIGS. 1 and 2

In accordance with the mode of the present process depicted in FIGS. 1and 2, a sheet of transparent insulating material, hereinafter termed atransfer sheet, is placed on a photoconductive surface to form acomposite. Referring specifically to FIG. 1, a composite 20 isschematically depicted, which comprises a transfer sheet 22 disposed ona photoconductive surface 24 of a photoconductor 25, in turn disposed ona grounded (at 26) conductor plate 28. The transfer sheet 22 can be athin, electrically, insulating film, for example, of plastic such asthin, smooth, uniform polyethylene terephthalate, polyethylene,polycarbonate, tetrafluoroethylene, polystyrene or the like film whichusually is used in a transparent form. In one mode in the presentprocess, more particularly described hereafter in connection with FIGS.5 and 6, the transfer sheet 22 need not be transparent. The transfersheet 22 is preferably dimensionally stable, thermally stable and wearresistant. A preferred thickness is, for example, 0.1-2 mil or the like.

The photoconductive surface 24 can be any suitable conventionalphotoconductive surface, such as cadmium sulfide, zinc sulfide, cadmiumsulfide and/or zinc sulfide in a binder resin, or the like. Zinc oxidepaper can also be used in this embodiment. Such paper comprises zincoxide particles in an insulating binder, such as polystyrene, phenolicresin, melamine formaldehyde resin, or the like, coated on a supportsuch as paper. The photoresponse of such paper apparently requires thedeposition of oxygen ions on its surface, obtained by direct charging,which is provided in the embodiment of FIGS. 1 and 2. However, in theother embodiments, no pre-charging of the photoconductor is provided andzinc oxide paper would have too low a photoresponse therefor. Theconductor plate 28 may be any suitable metal or metal coated glass,etc., for example, a steel plate, suitably grounded at 26 and bearingthe photoconductor 25 with its photoconductive surface 24 abutting thetransfer sheet 22, particularly the lower surface 30 thereof. The lowertransfer sheet surface 30 in all embodiments of the present process isuniformly electrostatically charged before application to thephotoconductive surface 24 and is in that condition during saidapplication and exposure, as shown, for example, in FIG. 1. Moreover, inthe embodiment depicted in FIG. 1, the photoconductive surface 24 hasalso been precharged electrostatically to the same polarity as that oftransfer sheet surface 30 and is in that state when assembled with thetransfer sheet 22 to form the composite 20. It is preferred for thisembodiment that the photoconductor surface 25 be of the n-p type.

It is important that there be no electrical discharge between thephotoconductive surface 24 and the transfer sheet surface 30 before orafter light exposure. Accordingly, the voltage potentials of thesesurfaces 24 and 30 are selected so as to avoid such discharge during thefollowing image formation.

Upon exposure of the photoconductive surface 24 to a pattern 31 oflight-directed through the transfer sheet 22, as shown in FIG. 1, avoltage equal and opposite to that on the charged transfer sheet surface30 appears on the light exposed areas of the photoconductive surface 24.As a result, the effective voltage difference between the illuminatedand non-illuminated areas on the photoconductive surface 24 is thealgebraic difference between the original and induced potentials. Forexample, if the original potentials on the photoconductive surface 24and transfer sheet surface 30 are each 300 volts negative beforeexposure, the effective voltage before exposure is 600 volts negative.Where the photoconductive surface 24 is illuminated, a positive 300volts is induced, effectively neutralizing the negative 300 volts on thetransfer sheet surface 30 so far as the electrical field above the freesurface 32 of the transfer sheet 22 is concerned. Thus, the effectivevoltage difference between the illuminated areas and unilluminated areasof the photoconductive surface 24 is 600 volts with or without thetransfer sheet 22 subsequently in place.

Thus, by utilizing a precharged transfer sheet 22 in the mannerdescribed in the present process, the voltage difference betweenilluminated and unilluminated areas of the photoconductive surface 24can be doubled. This doubling of voltage difference provides an increasein electrostatic contrast by a factor of two which automaticallyincreases resolution of the finished copy to be produced. It will benoted from FIG. 2 that in the areas of the free surface 32 of thetransfer sheet 22 which corresponds to the unexposed (unilluminated)areas of surface 24, an electrical potential of 600 volts negativeappears (assuming the original voltage total, as in the example above,was 600 volts negative) and in the areas of the free transfer sheetsurface 32, corresponding to exposed areas of the photoconductivesurface 24, the voltage potential is zero.

Accordingly, the free transfer sheet surface 32 can be developed withelectroscopic particles of any conventional type well known and adaptedto be attracted to and adhere to the free transfer sheet surface 32 inaccordance with the charge pattern thereon. A visible image is thusprovided on the free surface 32 of the transfer sheet corresponding tothe light pattern to which the photoconductive surface 24 has beenexposed, as per FIG. 1. Such development is carried out with thetransfer sheet 22 in place on the photoconductive surface 24 as shown inFIG. 1, after which the transfer sheet 22 can be separated from thephotoconductive surface 24. Fixing of the visible image on the transfersheet 22 can be performed as shown in FIG. 11, in any conventionalmanner, as by heat fusing resin-bearing toner particles 31 in place tothe transfer sheet 22 supported on a platen 33 beneath a heater 35 whichcan have any conventional form. The visible image can also first beprinted off onto a receiving sheet, such as plain bond paper or the likeand then fixed on the receiving sheet.

If the visible image obtained as described above is printed off so as toprovide the transfer sheet 22 with a toner-free surface 32, the transfersheet 22 can then be reapplied to the photoconductive surface 24 withits charged lower surface 30 in contact therewith and the free surface32 of the transfer sheet can be redeveloped with toner to provide asecond visible image identical to the first visible image describedabove. The redeveloped transfer sheet 22 can then be separated from thephotoconductive surface 24, as before, and printed off and fixed aspreviously described. It will be noted that there is no need to reexposethe photoconductive surface 24 to the light pattern before suchredevelopment takes place. Since the voltage potentials of thephotoconductive surface 24 and transfer sheet surface 30 still exist,the free transfer sheet surface 32 is provided with the same effectivevoltage potentials defining the same image pattern as before. Thus, alarge number of copies of the same image can be made consecutivelyutilizing only a single charge and exposure of the photoconductor 25,limited only by its dark decay rate.

Referring to FIG. 12, a device is shown which is identical to anembodiment depicted in above-referred to Gaynor U.S. Pat. No. 3,820,985,except for the additional incorporation of means for electrostaticallycharging the side of the transfer sheet which is to contact thephotoconductor surface and use of indicia to indicate charge. Suchapparatus enables the visible image to be printed off onto bond paperand fixed thereon. As stated in the Gaynor et al patent, a xerographicplate having a photoconductive surface 24' and a conductive backing 28'is arranged in the form of a cylindrical drum 14. The drum 14 is mountedfor rotation on a shaft 16 that is rotated at a predetermined speed bysuitable motive means (not shown). An endless belt 22' of lighttransmitting material is overlaid on a portion of photoconductivesurface 24' and is held in direct close contact with the photoconductivesurface 24' by means of suitable tensioning means (not shown) operativewith various rollers as required for subsequent operations. In thisparticular configuration, the belt 22' is led over a rubber transferroller 21 over an idler roller 23, and back onto the photoconductivesurface 24' of the drum 14. The drum 14 and belt 22' travel in acounterclockwise direction and a corona charging grid 25 is disposedadjacent the photoconductive surface 24' at a point prior to its contactwith the belt 22'. A discharge lamp 27 is disposed at a position priorto the disposition of the corona charging grid 25 and subsequent toseparation of the belt 22' from the photoconductive surface 24', allwith respect to the direction of travel of the belt 22' and drum 14. Adocument exposure station 29 is disposed to overlie a contact region ofthe belt 22' and photoconductive surface 24' and is followed in thecourse of travel of the drum by a toning station 31 which is alsodisposed adjacent a contact region between the belt 22' andphotoconductive surface 24'.

The toner is attracted to the belt 22' as a result of inducedelectrostatic forces through the belt and forms a toner image of thedocument on the belt 22'. The belt 22' can be then separated from thedrum surface 24' and may be led with its toner image, as indicated at33, to a toner transfer station 35.

At the image transfer station 35, the conductive roller 21 compressesthe toner bearing belt 22' into contact with a support sheet 37 which issandwiched between the belt 22' and a metallic roller 39. The supportsheet 37 can be a paper sheet or any desired support member to which atoner image will adhere. The metal roller 39 is formed with an axiallycentral heating rod 41, heated by means of a power source showndiagrammatically at 43, so that heat is applied through the supportsheet 37 to fuse the toner thereto. The support sheet 37 is fed from asupply 45 thereof by means of a pressure roller 47 actuated inregistration with travel of the belt 22' by a mechanism not shown. Asthe toner image is transferred, the support sheet 37 passes onto aconveyor belt 49 and from there into a receptacle 51.

In accordance with the present invention, and as above indicated, theside of the transfer sheet (belt) 22' which is to contact thephotoconductive surface 24' is electrostatically charged, indicatedschematically by electrostatic charge means 53, at a point prior tocontact with the photoconductive surface 24'.

It will be further noted that a separate, uncharged, transfer sheet 22can be used for the redevelopment step, if desired, without incurringloss of contrast. In this regard, the latent electrostatic image on thephotoconductive surface 24 after the original exposure described aboveconsists (as per the above example) of exposed positively charged areasexhibiting 300+ volts, whereas the unexposed areas of surface 24 exhibit300- volts. Thus, the 600 volt gradient is still present to provide theoriginal high contrast. The electric field above the free surface of anew, uncharged transfer sheet 22 will exhibit the same voltage gradientso that the new free surface 32 has an induced latent electrostaticimage of the same contrast as before, which surface can be developed asbefore.

Moreover, if a conventional biasing electrode is used while the tonerredevelopment step is carried out, positive or negative images on thefree transfer sheet surface 32 can be provided with the same tonerparticles, depending on the sign of the electrode potential employed andits magnitude. It will also be noted that, with or without the biasingelectrode, the redevelopment step with an uncharged transfer sheet iscarried out in a most favorable environment for very high image qualityreproduction, since charged toner particles are attracted during theredevelopment to image-bearing areas having charges opposite to that ofthe toner and are simultaneously repulsed from the background areas,i.e., non-image bearing areas of the free transfer sheet surface 32(areas bearing the same charge sign as the toner). This also enablespositive and negative images to be toner developed with equal facilityand quality using the same process but with different toners.

One should select the material constituting the transfer sheet 22 andthe photoconductive surface 24 in order to prevent discharge between thelower charged transfer sheet surface 30 and the photoconductor 25 duringoriginal exposure and development (and subsequent redevelopment usingthe original transfer sheet 22). For example, with a transfer sheet 22formed from polyethylene terephthalate and a photoconductor 25 formedfrom selenium, if the voltage gradient is kept below about 650 volts, nodischarge will occur. Similar maximum limits for voltage gradients applyto other combinations, and such limits are known to the art or arereadily determinable. See, in this regard, "Electrophotography", by R.M. Schaffert, Focal Press, New York, 1965. It will be further understoodthat the original voltages on the charged transfer sheet surface 30 andthe photoconductive surface 24 need not be numerically the same in orderto provide the desired results.

FIGS. 3 and 4

Now referring to FIGS. 3 and 4, a composite 120 is shown in use duringand after exposure. The composite 120 is identical to the composite 20of FIGS. 1 and 2 except that, in this instance, in contrast to the modeshown in FIGS. 1 and 2, the photoconductive surface 124 is in theuncharged state (therefore, one would not use zinc oxide paper as thephotoconductor). The lower surface 130 of the transfer sheet is,however, charged, as in FIGS. 1 and 2. For the mode shown in FIGS. 3 and4, it is preferred that a p-type photoconductor 125 be employed when theelectrostatic charge on transfer sheet surface 130 has a negative sign.Suitable examples of p-type photoconductors include: selenium,selenium-tellurium alloys, and polyvinylcarbazole-trinitrofluorenonemixtures. When the charge on the transfer sheet surface 130 is positive,an n-type photoconductor can be used, such as: cadmium sulfide orcadmium selenide. Of course, a n-p type of photoconductor can be usedregardless of the polarity of the charge on the transfer sheet surface130.

Conventional electrostatic charging devices such as a corona chargingdevice, widely used in electrophotographic copying machines, arerelatively expensive, inconvenient, sources of maintenance problems,potential hazards and space occupiers. Consequently, elimination of theneed for such devices represents a substantial advance in the art. Inthe mode illustrated in FIGS. 3 and 4, such a need is eliminated becauseno charging of the photoconductive surface 124 is required. Moreover,although the transfer sheet surface 130 must be charged, it can beprecharged in advance of use. Polymeric insulators such as polyethyleneterephthalate, polystyrene, tetrafluoroethylene, polyethylene and thelike can easily be precharged to the relatively low electrostaticpotentials required and can retain their charges for substantial lengthsof time. They can also be folded or coil wrapped in thin film formwithout charge transfer from one surface to another, rendering themideal for stable precharged transfer sheets and rolls of compactconfiguration.

Moreover, it is possible to employ as a transfer sheet 122 an electretwhich would ensure the existence of the charge for a very long time.Electrets are permanently electrified substances well known in the art.They are usually made from polar dielectric materials whose moleculesare aligned in an imposed electrical field. Polar plastic materials suchas selected vinyls, acetals, acrylics, polyesters and silicones, amongothers, are well known. Polyethylene terephthalate, for example, can beformed into an electret by polarization at about 85° C.

The magnitude of the voltage charge on the lower surface 130 of thetransfer sheet 122 (or through the transfer sheet if it is an electret)must be kept below the threshold which would permit charge transfer tothe uncharged photoconductive surface 24. Thus, the voltage gradient, asdescribed for the mode of FIGS. 1 and 2, must be kept below about 650volts, meaning that the original voltage on the charged transfer sheetsurface 130 should not be in excess of about 325 volts.

Once the charged transfer sheet surface 130 is in place on the unchargedphotoconductive surface 124, (the photoconductor 125 being disposed on agrounded base conductor 128) the composite 120 is exposed to a lightpattern through the transparent transfer sheet 122 so that the lightexposes the photoconductor 125. In the areas of the photoconductivesurface 124 exposed to light, a potential equal in magnitude andopposite in sign to that on the charged transfer sheet is induced on thephotoconductive surface 124. Accordingly, the electrical field above thecomposite in light exposed areas is zero and toner carrying a positivecharge will not deposit. In the unexposed areas of the composite 120,there is an electrical field above the free transfer sheet surface 132because a similar compensating voltage has not been induced in thosecorresponding areas of the underlying photoconductive surface 124.Consequently, a visible image corresponding to the light pattern whichimpinged on the photoconductive surface 124 can be produced on the freetransfer sheet surface 132 by toner development in the manner previouslydescribed. Such development is conducted with the transfer sheet 122 inplace on the photoconductor 125, after which the transfer sheet 122 canbe separated therefrom and the visible image is fixed, as previouslydescribed, with or without intervening transfer of the visible image toa receiving sheet.

The latent electrostatic image induced in the photoconductive surface124 will remain for a time proportional to the inherent dark decay rateof the photoconductive surface 124 so that additional image copies canbe made (before such decay) in the manner described for the mode ofFIGS. 1 and 2, i.e., reapplication of the charged transfer sheet 122 tothe photoconductive surface 124, redevelopment in place (withoutreexposure), removal of the developed transfer sheet 122, print-off ofthe visible image and repetition of this cycle. The voltage on thecharged transfer sheet surface 130 remains constant within its inherentelectrical time constant limits.

FIGS. 5 and 6

Photoconductors which are not good photoinsulators and have high darkdecay rates nevertheless can be effectively used in accordance with thepresent process by the procedure exemplified in FIGS. 5 and 6. Theextreme light sensitivity and broad spectral response of certain of suchmaterials gives them advantages in use which can be utilized by thepresent procedure. They can be successfully used to produce very highresolution copies of very good quality. Examples of such photoconductivematerials include: doped cadmium sulfide, cadmium selenide, cadmiumtelluride and selenium telluride.

Such materials are utilized generally in the mode of FIGS. 3 and 4,modified as shown in FIGS. 5 and 6. Thus, and uncharged surface 224 of aphotoconductor 225, disposed on a conductor 228 grounded at 226, iscontacted with the electrostatically charged surface 230 of a transfersheet 222. The transfer sheet 222 is identical to the transfer sheet 122of FIGS. 3 and 4 and the sheet 22 of FIGS. 1 and 2, but, in contrast tothe mode of those Figures, the transfer sheet 222 can be, but need notbe, transparent. However, in this embodiment the conductor 228, must betransparent for simultaneous exposure and development. This can beaccomplished, as shown in FIG. 6, by exposing the photoconductor 225 toa light pattern through the transparent plate 228 while simultaneouslyapplying charged toner developer to the free surface 232 of the transfersheet 222.

Although the electrostatic image induced on the photoconductive surface224 decays very rapidly, it can easily be captured by toner which issimultaneously applied to the free transfer sheet 232. The resultingtoner image can then be transferred to a receiving sheet and fixed, orit can be fixed on the transfer sheet 222, if desired. The high decayrate of the photoconductive surface 224 prevents multiple copies of suchimage from being made by the redevelopment procedure described withrespect to FIGS. 3 and 4. However, further copies can be made byrepeating the entire procedure, including reexposure of thephotoconductive surface 224. Thus, high speed photoconductors previouslyunusable in photocopying processes can be utilized in the presentprocess to provide high quality copies.

FIGS. 7, 8 and 9

In accordance with another embodiment of this invention, electrostaticimages are recorded directly on the free surface of the transfer sheet.Such images can be developed and duplicated without the presence of thephotoconductive surface, thus affording modular operation and greaterversatility of the present process to permit various types of machinedesigns. Moreover, electrostatic contrast and resolution can beincreased and the duration of the image can be made substantially longerthan the decay time of the photoconductive surface so that many copiescan be made therefrom.

In accordance with this embodiment, while the transfer sheet is incontact with the photoconductive surface, a real electrostatic image canbe produced on the surface of the transfer sheet, thus allowing multiplecopying. This is accomplished by charging the surface of the transfersheet to a constant voltage simultaneously with exposure of thephotoconductive surface. If such charging is carried out in the mode ofFIGS. 5 and 6, the need to simultaneously expose and develop isobviated, and developing can occur at any time after exposure, since theimage so produced on the free transfer sheet surface 232 (332 in FIG. 7)is longlasting. Therefore, the photoconductive surface can be lightirradiated either through the supporting transparent conductor 228 orthrough the transparent transfer sheet.

A constant voltage corona device 334 is used for charging as shown inFIG. 8, parts A and B, which device is known in the art as a variablecurrent or variable charge deposition device. The amount of chargedeposited depends on the sign and magnitude of the voltage or thecapacitance which the device detects. Thus, if voltage detected is ofthe same sign as the charges being deposited by the device, then lesscharge will accumulate in the high voltage area read out by the device.If the device deposits charges of opposite sign to the voltage beingdetected, more charge will accumulate in the higher voltage areas. Highcapacitance areas will acquire more charge from the device than lowcapacitance areas in order to attain the same voltage potential.Electrostatic contrast enhancement is possible with such a device.

Referring particularly to FIG. 7, a composite 320 is provided comprisinga conductor plate 328, photoconductor 325 having a photoconductivesurface 324, and a transfer sheet 322 having its lower surface 330 incontact with, but removable from, the photoconductive surface 324. Thephotoconductor 325 or transfer sheet 322 or both, has a uniform (in thiscase, negative) charge in its contacting surface. The exposure stepshown for the composite 320 results in the previously described voltageand capacitance differences between the exposed and unexposed areas ofthe photoconductive surface 324 (and corresponding areas of transfersheet surface 332). By the use of a constant voltage-variable currentcorona charging device 334, as shown in FIG. 8, parts A and B, thedifferences noted above are recorded on the free surface 332 of thetransfer sheet 322 as charge density differences, thereby creating areal, longlasting electrostatic image independent of the decay rate ofthe photoconductive surface 324. In effect, the internal voltagedifferences between the light exposed and the unexposed areas of thecomposite 320 are recorded on the free transfer sheet surface 332 ascharge density differences. Depending on the polarity of charging device334, either a positive or negative real electrostatic image can berecorded on the free transfer sheet surface 332 relative to a onepolarity toner. For simplicity, FIGS. 8A and 8B show only chargedifferentials. FIG. 8, part A, represents the charge density build-uputilizing a free transfer sheet surface 332 processed according to themode of FIGS. 1 and 2, while FIG. 8, part B, represents the chargedensity build-up utilizing a free transfer sheet surface 332 processedaccording to the mode of FIGS. 3 and 4.

So long as the transfer sheet 322, after use of the charging device 334as per FIG. 8, remains in contact with the photoconductive surface 324and the composite 320 is kept in the dark, the free transfer sheetsurface 332 does not exhibit apparent voltage differences and, thuscannot be toned (except in the case of the high decay ratephotoconductor as referred to in FIGS. 5 and 6, after decay). Thus, itis necessary to either remove the transfer sheet 322 from thephotoconductive surface 324 and place it on a grounded conductor withthe lower transfer sheet surface 330 contacting the same before toningthe free surface 332 can take place, or to discharge the photoconductivesurface 324 by exposing it to light and then tone the free transfersheet 332 in place in the composite 320. It is only when either of thesesteps is taken that the charge differences on the free transfer sheetsurface 332 are converted into real voltage differences and toning canproceed.

FIG. 9, parts A and B, depict toning of the free transfer sheet surface332 shown in FIG. 8, parts A and B, respectively, after application ofthe lower transfer sheet surface 330 to a conductor 336 grounded at 338.Such toning provides a visible image of high image density andresolution. Moreover, the procedure of FIGS. 7, 8 and 9 is susceptibleto an increase in effective photographic speed. However, since thecharged toner particles contact the real electrostatic image directly,some charge removal occurs and the numbers of copies which can be madefrom the real electrostatic image is limited unless the transfer sheet322 is an electret or a material with electret stability. Accordingly,in many instances, this mode will have particular application where thetransfer sheet 322 is to have the visible image affixed directlythereto, as in microimagery, x-ray, transparencies and photographicprints.

Modification of the mode of FIGS. 5 and 6 to encompass realelectrostatic image recording on the free transfer sheet surface can bemade, as previously described. Thus, simultaneous exposure of thephotoconductive surface and developing of the free transfer sheetsurface are obviated so long as exposure and constant voltage chargingare simultaneous. Furthermore, a precharged transfer sheet is notrequired. Charge carriers generated in light exposed photoconductorareas produce capacitance differences. The resulting real electrostaticimage can be developed with the transfer sheet 322 in or out of contactwith the photoconductive surface 324, since the charge on thephotoconductive surface 324 is automatically dissipated beforedevelopment (due to the high dark decay rate). However, thephotoconductor 325 should be grounded or the transfer sheet 322 shouldbe placed on the grounded conductor 336 as shown in FIG. 9, parts A andB.

FIG. 10

In this embodiment, the real electrostatic image formed by theprocedures set forth above is used to make a very large number of copiesby applying the side 332 bearing the real electrostatic image to ablocking layer 340 on the grounded conductor 336, rather than placingthe lower side 330 of the transfer sheet 332 thereon. The uncharged, oruniformly charged, lower side 330 is now free and exposed and can bedeveloped with toner 342. The blocking layer 340 is very thin and maycomprise, for example: aluminum oxide or thin plastic films such aspolyethylene or polystyrene. The blocking layer 340 is not critical ifthe voltage on the transfer sheet is below the breakdown voltage, but ispreferred to provide good contrast. The blocking layer 340 preventsdrain-off of the charge from charged transfer sheet surface 332 throughthe conductor 336.

The real electrostatic image on the transfer sheet surface 332 induces acorresponding latent electrostatic image on the uncharged, or uniformlycharged, surface 330, which image can be toner developed, transferred toreceiving sheets (copy sheets) and fixed, redeveloped, etc. The realelectrostatic image is strong and protected from dissipation, since thetransfer sheet 322 is an excellent insulator. Accordingly, the lifetimeof the real electrostatic image is very great and toner developing ofthe side 330 does not affect its durability. Extension of the durabilityof the image to be reproduced is thus accomplished in a simple effectiveway which readily lends itself to multiple copying and modular layout,with separate exposing and developing-duplicating areas, for moreeffective and simplified machine construction and operation. A reversalmirror should be used during exposure to produce proper copy imageorientation.

In each of the foregoing embodiments, it is preferred that the thininsulating film have high lateral electrical resistivity, at least 10¹³ohms/square of surface, to prevent image spread. High bulk resistivity,at least 10¹⁵ ohm-cm, is desirable to assure localized latentelectrostatic images with long electrical lifetimes so that number ofcopies is not limited unnecessarily.

It will also be understood that due to the inherent ability of variousmodes of the present process to produce multiple copies of the sameimage from a single exposure, various techniques can be applied for thesequential application of toners of various colors to providemulti-colored copies. It will be further understood that the variousmodes of the present process are readily adaptable for use with avariety of equipment components heretofore utilized in the electrostaticcopying art. Moreover, the present process, while simple, rapid andeffective, produces copies of superior contrast and resolution. Otheradvantages are as set forth in the foregoing.

Various changes, modifications and alterations can be made in thepresent process, its steps and parameters. All such changes,modifications and alterations as are within the scope of the appendedclaims form part of the present invention.

I claim:
 1. An electrostatic reproduction device, comprising:aphotoconductive surface; means for uniformly charging saidphotoconductive surface to a predetermined polarity; means for chargingone side only of a sheet of insulating transfer material to saidpolarity; means for thereafter temporarily placing said charged sheet ofinsulating transfer material on said photoconductive surface with saidone side thereof in contact with said photoconductive surface; means forexposing said photoconductive surface to a light pattern with saidtransfer material in place; means for developing said transfer sheet toform a visible image thereon; and means for fixing said visible image.2. The device of claim 1 in which said fixing means comprises meansfixing said visible image on said transfer sheet.
 3. The device of claim1 including means for transferring said visible image from said transfersheet to a receiver sheet, said fixing means comprising means for fixingsaid visible image on said receiver sheet.
 4. The device of claim 1including means for removing said transfer sheet from saidphotoconductive surface.
 5. The device of claim 4 including means forreapplying said transfer sheet to said photoconductive surface aftersaid removal thereof.
 6. An electrostatic reproduction device,comprising:a photoconductive surface; means for uniformly charging saidphotoconductive surface to a predetermined polarity; means for chargingone side only of a sheet of insulating transfer material to saidpolarity; means for thereafter temporarily placing said charged sheet ofinsulating transfer material on said photoconductive surface with saidone side thereof in contact with said photoconductive surface; means forexposing said photoconductive surface to a light pattern with saidtransfer material in place; means for charging the other side of saidtransfer sheet to a constant voltage; means for developing said transfersheet to form a visible image thereon; and means for fixing said visibleimage.
 7. An electrostatic reproduction device, comprising:aphotoconductive surface; means for charging said photoconductivesurface; means for charging one side only of a sheet of insulatingtransfer material; means for temporarily placing said one side of saidsheet of insulating transfer material on said photoconductive surface;means for exposing said photoconductive surface to a light pattern;means for charging the other side of said applied transfer sheet to aconstant voltage; means for developing said transfer sheet to form avisible image thereon; and means for fixing said visible image.